1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a tandem type image forming apparatus that forms a toner image using electrophotography.
2. Description of the Related Art
Tandem type image forming apparatuses capable of improving a printing speed have been used as multi-color image forming apparatuses, such as, color printers, color copiers, color facsimiles, and the like. In tandem type image forming apparatuses, a plurality of image forming portions are disposed on a transfer carrier belt or an intermediate transfer belt so that toner images corresponding to images of separated colors are sequentially transferred onto transfer paper absorbed by the transfer carrier belt or directly onto the intermediate transfer belt and overlapped thereon, thereby forming a multi-color toner image.
Since tandem type image forming apparatuses include a plurality of image forming portions as described above, a minimization of each of the image forming portions is strongly required to reduce sizes of the apparatuses. Thus, cleanerless systems having no cleaning means for removing transferred toner remaining on a photosensitive conductor have been proposed.
For example, Japanese Patent Publication No. 2001-337503 (refer to pages 4 through 7 and FIG. 1 thereof) discloses a cleanerless type color image forming apparatus that sequentially transfers yellow, cyan, magenta, and black colors onto an intermediate transfer belt. The color image forming apparatus includes a brush that agitates residual toner attached onto photoconductive drums to weaken adhesion of the residual toner to the photoconductive drums.
Japanese Patent Publication No. hei 5-2289 (refer to page 2 and FIGS. 1 and 4 thereof) discloses a tandem type image forming method adopting such a cleanerless system, in which a predetermined voltage is applied to a transfer device after a typical image forming process or a jam removal operation and toner accumulated in a memory removal member is returned to an image carrier.
However, the above-described conventional image forming apparatuses have several problems. One problem in the image forming apparatus disclosed in Japanese Patent Publication No. 2001-337503 is that the adhesion of the toner is weakened by the brush for collecting toner. Additionally, toner agitation intensity higher than mono-color image forming apparatuses is required because of the large generation of residual toner. Hence, a greater burden is imposed upon the photoconductive drums than in the mono-color image forming apparatuses. Particularly, a life span of a photoconductive drum that forms a black toner image is shortened because it is frequently used.
Another problem exists when the image forming method disclosed in Japanese Patent Publication No. hei 5-2289 is applied to tandem type image forming apparatuses. A higher voltage is applied to the memory removal member located at a lower course of a toner stream, which accumulates reversely transferred toner, than to an upper course of the toner stream to return residual toner to the image carrier. Alternatively, the residual toner may be returned to the image carrier by applying a predetermined voltage for a longer period of time to the memory removal member than to the upper course of the toner stream. In other words, compared with a mono-color control case, a high-voltage disclosure condition is strict, the amount of charge products attached to the image carrier increases, or the reliability of an internal pressure of the image carrier is degraded. The charge products are produced due to discharge from the transfer device. Consequently, the quality of an image is degraded, and the durability of the image carrier becomes shorter.